With mobile electronic devices such as mobile phones, personal data assistants (PDA), laptop computers, and camcorders becoming more multi-functional, power consumption and usage time for these devices increased. To provide for these increases, high energy density is strongly desired for the batteries to be mounted on these devices. Currently, lithium secondary batteries are mainly used for those devices. However, the energy density of lithium secondary batteries are predicted to reach its limit at about 600 Wh/L in around the year 2006, and for replacement, polymer electrolyte fuel cells (PEFC) are expected to be in practical use earlier.
Among fuel cells, a direct methanol fuel cell (also simply referred to as DMFC hereinafter), is gaining attention and researches and developments are actively conducted for DMFC. In DMFC, a fuel, namely methanol or an aqueous methanol solution, is supplied to the inside of the cell without reforming the fuel to hydrogen, and oxidized at an electrode for acquiring electricity. Reasons for DMFC to gain attention may be the following, just to name a few: an organic fuel has a high theoretical energy density and is easy to store, and further, a direct methanol fuel cell system can be easily simplified.
A cell of a DMFC is structured to have the following: a membrane electrode assembly (MEA) obtained by sandwiching a polymer electrolyte membrane with an anode (fuel electrode) and a cathode (air electrode) each having a catalyst layer and a diffusion layer; and a pair of separators sandwiching both sides of the MEA. A fuel, namely methanol or an aqueous methanol solution, is supplied to an anode, and an air is supplied to a cathode to obtain electricity.
An electrode reaction in a DMFC is illustrated in the following:Anode: CH3OH+H2O→CO2+6H++6e−  (1)Cathode: (3/2)O2++6H++6e−→3H2O  (2)
The formulae (1) and (2) above show that in the anode, methanol and water are reacted to produce carbon dioxide, protons, and electrons; the protons reach the cathode via the polymer electrolyte membrane; and in the cathode, oxygen, the protons, and the electrons passed through the external circuit, are bonded to produce water.
However, there are some problems in practical usage of DMFC. One of the problems is related to discharge of the reaction product, i.e., the carbon dioxide gas. Carbon dioxide generated in the anode passes through the anode-side diffusion layer, reaches into the flow path of the separator, and finally is discharged to the outside via the flow path. At this time, the generated carbon dioxide partially remains in the diffusion layer to inhibit the fuel to diffuse into the catalyst layer, and gradually accumulates to create large bubbles. Then, the bubbles push the fuel out from the micropores of the diffusion layer, thereby causing the fuel supply for the catalyst layer to be insufficient, and a portion of unused fuel to be discharged to the outside. As a result, the electricity production might drop greatly at a high current density side.
As a method to solve such problems, there has been proposed to provide a liquid fuel flow path and an exhaust flow path independently (i.e., completely separated), as well as a diffusion layer which has liquid permeability but hardly permeates gas to face the liquid fuel flow path, and a diffusion layer having gas permeability to face the exhaust flow path in the anode side separator (i.e., Japanese Laid-Open Patent Publication No. 2002-175817).
However, the above conventional technique is yet to be improved, in terms of providing a direct methanol fuel cell having sufficient electricity production ability without reducing fuel usage efficiency.
According to the above technique, the problem in carbon dioxide gas discharge is possibly solved. However, since the completely separated fuel flow path and exhaust flow path are provided at the anode side separator, when the diffusion layer is low in fuel permeability, the fuel amount to be supplied to the region in the surface of the catalyst layer facing the exhaust flow path becomes insufficient, causing a decrease in the output.
Thus, to solve such conventional problems as mentioned in the above, the present invention aims to provide a direct methanol fuel cell with excellent electricity production ability in which the amount of the fuel supply for the catalyst layer is secured, and the produced carbon dioxide gas is further reliably discharged.